The application generally relates to using an optocoupler in applications having high voltage and/or high power circuits. The application relates more specifically to a system and method for using optocouplers to provide electrical isolation in gate driver circuits or boards in variable speed drives, where large common mode voltage transients occur between the input and output of the optocoupler.
An optocoupler includes a photo light-emitting diode (LED) and a photo-detector, or phototransistor between the LED and photo-detector. The optocoupler can be used for isolating two circuits. When an optocoupler is used in gate driver applications for high voltage and/or high power circuits, the isolating sides of the optocoupler may be exposed to high voltage, common mode transients.
An optocoupler has a common mode immunity rating that may be in the range of tens of kilovolts per microsecond (KV/μs). If common mode voltage transients exceed the common mode immunity rating, the optocoupler may fail. Optocoupler failure may result in the photo LED being switched on or off unintentionally. In sensitive applications, for example, a gate driver of a variable speed drive (VSD), unintentional switching of an optocoupler may result in a destructive shoot-through of the DC link voltage through the gated insulated gate bipolar transistor (IGBT).
In an optocoupler, the photo LED and photo-detector are not uncoupled electrically. Capacitive coupling may occur between the photo LED and the photo-detector. The coupling capacitance may be represented as a capacitor connected between each of the anode and cathode of the photo LED, and the output of the optocoupler. Thus, a large transient in the common mode voltage can result in current flowing to or from the anode or the cathode of the photo LED. The additional current due to common mode voltage transients can cause a photo LED to malfunction, e.g., to unintentionally turn on or off the photo detector.
Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.